In the fabrication of silicon integrated circuits, the continuing increase in the number of devices on a chip and the accompanying decrease in the minimum feature sizes have placed increasingly difficult demands upon many of the many fabrication steps used in their fabrication including depositing layers of different materials onto sometimes difficult topologies and etching further features within those layers.
Oxide etching has presented some of the most difficult challenges. Oxide is a somewhat generic term used for silica, particularly silicon dioxide (SiO.sub.2) although slightly non-stoichiometric compositions SiO.sub.x are also included, as is well known. The term oxide also covers closely related materials, such as oxide glasses including borophosphosilicate glass (BPSG). Some forms of silicon oxynitride are considered to more closely resemble a nitride than an oxide. Small fractions of dopants such as fluorine or carbon may be added to the silica to reduce its dielectric constant. Oxide materials are principally used for electrically insulating layers, often between different levels of the integrated circuit. Because of the limits set by dielectric breakdown, the thickness of the oxide layers cannot be reduced to much below 0.5 to 1 .mu.m. However, the minimum feature sizes of contact and via holes penetrating the oxide layer are being pushed to well below 0.5 .mu.m. The result is that the holes etched in the oxide must be highly anisotropic and must have high aspect ratios, defined as the depth to the minimum width of a hole. A further problem arises from the fact that the underlying silicon may be formed with active doped regions of thicknesses substantially less than the depth of the etched hole (the oxide thickness). Due to manufacturing variables, it has become impossible to precisely time a non-selective oxide etch to completely etch through the silicon oxide without a substantial probability of also etching through the underlying active silicon region.
The anisotropy can be achieved in dry plasma etching in which an etching gas, usually a fluorocarbon, is electrically excited into a plasma. The plasma conditions may be adjusted to produce highly anisotropic etching in many materials. However, the anisotropy should not be achieved by operating the plasma reactor in a pure sputtering mode in which the plasma ejects particles toward the wafer with sufficiently high energy that they sputter the oxide. Sputtering is generally non-selective, and high-energy sputtering also seriously degrades semiconducting silicon exposed at the bottom of the etched contact hole.
In view of these and other problems, selective etching processes have been developed which depend more upon chemical effects. These processes are often described as reactive ion etching (RIE). A sufficiently high degree of selectivity allows new structures to be fabricated without the need for precise lithography for each level.
An example of such an advanced structure is a self-aligned contact (SAC), illustrated in the cross-sectional view of FIG. 1. A SAC structure for two transistors is formed on a silicon substrate 2. A polysilicon gate layer 4, a tungsten silicide barrier and glue layer 6, and a silicon nitride cap layer 8 are deposited and photolithographically formed into two closely spaced gate structures 10 having a gap 12 therebetween. Chemical vapor deposition is then used to deposit onto the wafer a substantially conformal layer 14 of silicon nitride (Si.sub.3 N.sub.4), which coats the top and sides of the gate structures 10 as well as the bottom 15 of the gap 12. In practice, the nitride deviates from the indicated stoichiometry and may have a composition of SiN.sub.x, where x is between 1 and 1.5. The nitride acts as an electrical insulator. Dopant ions are ion implanted using the gate structures 10 as a mask to form a self-aligned p-type or n-type well 16, which acts as a common source for the two transistors having respective gates 10. The drain structures of the transistors are not illustrated.
An oxide field layer 18 is deposited over this previously defined structure, and a photoresist layer 20 is deposited over the oxide layer 18 and photographically defined into a mask so that a subsequent oxide etching step etches a contact hole 22 through the oxide layer 18 and stops on the portion 24 of the nitride layer 14 underlying the hole 22. It is called a contact hole because the metal subsequently deposited into the contact hole 22 contacts silicon rather than a metal layer. A post-etch sputter removes the nitride portion 24 at the bottom 15 of the gap 12. The silicon nitride acts as an electrical insulator for the metal, usually aluminum, thereafter filled into the contact hole 22.
Because the nitride acts as an insulator, the SAC structure and process offer the advantage that the contact hole 22 may be wider than the width of the gap 12 between the gate structures 10. Additionally, the photolithographic registry of the contact hole 22 with the gate structures 10 need not be precise. However, to achieve these beneficial effects, the SAC oxide etch must be highly selective to nitride. That is, the process must produce an oxide etch rate that is much greater than the nitride etch rate. Numerical values of selectivity are calculated as the ratio of the oxide to nitride etch rates. Selectivity is especially critical at the corners 26 of the nitride layer 14 above and next to the gap 12 since the corners 26 are the portion of the nitride exposed the longest to the oxide etch. Furthermore, they have a geometry favorable to fast etching that tends to create facets at the corners 26.
Furthermore, increased selectivity is being required with the increased usage of chemical mechanical polishing (CMP) for planarization of an oxide layer over a curly wafer. The planarization produces a flat oxide surface over a wavy underlayer substrate, thereby producing an oxide layer of significantly varying thickness. As a result, the time of the oxide etch must be set significantly higher, say by 100%, than the etch of the design thickness to assure penetration of the oxide. This is called over etch, which also accounts for other process variations. However, for the regions with a thinner oxide, the nitride is exposed that much longer to the etching environment.
Ultimately, the required degree of selectivity is reflected in the probability of an electrical short between the gate structures 10 and the metal filled into the contact hole 22. The etch must also be selective to photoresist, for example at facets 28 that develop at the mask corners, but the requirement of photoresist selectivity is not so stringent since the photoresist layer 20 may be made much thicker than the nitride layer 14.
In the future, the most demanding etching steps are projected to be performed with high-density plasma (HDP) etch reactors. Such HDP etch reactors achieve a high-density plasma having a minimum average ion density of 10.sup.11 cm.sup.-3 across the plasma exclusive of the plasma sheath. Although several techniques are available for achieving a high-density plasma such as electron cyclotron resonance and remote plasma sources, the commercially most important technique involves inductively coupling RF energy into the source region. The inductive coil may be cylindrically wrapped around the sides of chamber or be a flat coil above the top of the chamber or represent some intermediate geometry.
An example of an inductively coupled plasma etch reactor is the IPS etch reactor, which is also available from Applied Materials and which by Collins et al. describe in U.S. patent application, Ser. No. 08/733,544, filed Oct. 21, 1996 and in European Patent Publication EP-840,365-A2. As shown in FIG. 2, a wafer 30 to be processed is supported on a cathode pedestal 32 supplied with RF power from a first RF power supply 34. A silicon ring 36 surrounds the pedestal 32 and is controllably heated by an array of heater lamps 38. A grounded silicon wall 40 surrounds the plasma processing area. A silicon roof 42 overlies the plasma processing area, and lamps 44 and water cooling channels 46 control its temperature. The temperature-controlled silicon ring 36 and silicon roof 42 may be used to scavenge fluorine from the fluorocarbon plasma. For some applications, fluorine scavenging can be accomplished by a solid carbon body, such as amorphous or graphitic carbon, or by other non-oxide silicon-based or carbon-based materials, such as silicon carbide.
Processing gas is supplied from one or more bottom gas feeds 48 through a bank of mass flow controllers 50 under the control of a system controller 52, in which is stored the process recipe in magnetic or semiconductor memory. Gas is supplied from respective gas sources 54, 56, 58. The conventional oxide etch recipe uses a combination of a fluorocarbon or hydrofluorocarbon and argon. Octafluorocyclobutane (C.sub.4 F.sub.8) and trifluoromethane (CHF.sub.3) are popular fluorocarbons, but other fluorocarbons, hydrofluorocarbons, and combinations thereof are used.
An unillustrated vacuum pumping system connected to a pumping channel 60 around the lower portion of the chamber maintains the chamber at a preselected pressure.
In the used configuration, the silicon roof 42 is grounded, but its semiconductor resistivity and thickness are chosen to pass generally axial RF magnetic fields produced by an inner inductive coil stack 62 and an outer inductive coil stack 64 powered by respective RF power supplies 66, 68.
Optical emission spectroscopy (OES) is a conventional monitoring process used for end-point detection in plasma etching. An optical fiber 70 is placed in a hole 72 penetrating the chamber wall 40 to laterally view the plasma area 74 above the wafer 30. An optical detector system 76 is connected to the other end of the fiber 70 and includes one or more optical filters and processing circuitry that are tuned to the plasma emission spectrum associated with the aluminum or copper species in the plasma. Either the raw detected signals or a trigger signal is electronically supplied to the controller 52, which can use the signals to determine that one step of the etch process has been completed as either a new signal appears or an old one decreases. With this determination, the controller 52 can adjust the process recipe or end the etching step.
The IPS chamber can be operated to produce a high-density or a low-density plasma. The temperature of the silicon surfaces and of the wafer can be controlled. The bias power applied to the cathode 42 can be adjusted independently of the source power applied to the coils 62, 64.
It has become recognized, particularly in the use of HDP etch reactors, that selectivity in an oxide etch can be achieved by a fluorocarbon etching gas forming a polymer layer upon the non-oxide portions, thereby protecting them from etching, while the oxide portions remain exposed to the etching environment. It is believed that the temperature controlled silicon ring 36 and roof 42 in the reactor of FIG. 2 control the fluorine content of the polymer, and hence its effectiveness against etching by the fluorocarbon plasma, when the polymer overlies a non-oxide. However, this mechanism seems to be responsible for at least two problems if high selectivity is being sought. If excessive amounts of polymer are deposited on the oxide or nitride surfaces in the contact hole being etched, the hole can close up and the etching is stopped prior to complete etching of the hole. This deleterious condition is referred to as etch stop.
Further, the chemistry may be such that the polymer formation depends critically upon the processing conditions. It may be possible to achieve high selectivity with one set of processing conditions, but very small variations in those parameters may be enough to substantially reduce the selectivity on one hand or to produce etch stop on the other. Such variations can occur in at least two ways. The conditions at the middle of the wafer may vary from those at the center. Furthermore, the conditions may change over time on the order of minutes as the chamber warms up or on the order of days as the equipment ages or as chamber parts are replaced. It is felt that hardware can be controlled to no better than .+-.5 or 6%, and a safety margin or 3 to 6 is desired. Mass flow controllers 46 are difficult to control to less than .+-.1 sccm (standard cubic centimeter per minute) of gas flow so gas flows of any constituent gas of only a few sccm are prone to large percentage variations.
These factors indicate that a commercially viable etch process must have a wide process window. That is, moderate variations in such parameters as gas composition and chamber pressure should produce only minimal changes in the resultant etching.
Several oxide etch processes have been proposed which rely upon higher-order hydrogen-free fluorocarbons and hydrofluorocarbons, both generically referred to as fluorocarbons. Examples of higher-order fluorocarbons are fluoroethane, fluoropropane, and even fluorobutane, both in its linear and cyclic forms. In U.S. Pat. No. 5,423,945, Marks et al. disclose an oxide etch selective to nitride using C.sub.2 F.sub.6 in an HDP etch reactor having a thermally controlled silicon surface. Later process work with the IPS chamber of FIG. 2 has emphasized C.sub.4 F.sub.8 as the principal etchant species. Wang et al. have disclosed the use of fluoropropanes and fluoropropylenes, e.g., C.sub.3 F.sub.6 and C.sub.3 H.sub.2 F.sub.6, in U.S. patent applications, Ser. Nos. 08/964,504 and 09/049,862, filed Nov. 5, 1997 and Mar. 27, 1998 respectively. The two examples of fluorocarbons have F/C ratios of 2, as does C.sub.4 F.sub.8, and some researchers, including Yanagida in U.S. Pat. No. 5,338,399, believe this value produces the best passivating polymer. We have observed, however, that the etching profile cannot be controlled with C.sub.3 H.sub.2 F.sub.6.
If possible, it is desirable to use the already widely available fluoromethanes, which include carbon tetrafluoride (CF.sub.4), trifluoromethane (CHF.sub.3), difluoromethane (CH.sub.2 F.sub.2), and monofluoromethane (CH.sub.3 F). Hung et al. in U.S. patent application, Ser. No. 08/956,641, filed Oct. 23, 1997, suggest the use of CHF.sub.3 and CH.sub.2 F.sub.2. We have observed that this last combination is insufficiently selective, indicating poor polymer formation.
Although octafluorocyclobutane (C.sub.4 F.sub.8) remains the most popular oxide etching gas, we observe that it suffers from too narrow a process window. Furthermore, although C.sub.4 F.sub.8 is known to provide selectivity at the bottom of the etching hole, it provides little sidewall passivation, which is required for the desired vertical profiles. Also, C.sub.4 F.sub.8 has a boiling point of +1.degree. C., which is considered somewhat high, especially in view of a trend to operate with very cold cathodes. Often carbon monoxide (CO) is added to C.sub.4 F.sub.8 to increase the sidewall passivation as well as increase general nitride selectivity. However, CO is not only toxic, it also forms carbonyls with nickel and iron in gas cylinders and supply lines. The carbonyls are believed to contaminate wafers. For these reasons, the use of CO is not preferred.
The two approaches using alternatively the fluoromethanes and hexafluoropropane (C.sub.3 H.sub.2 F.sub.6) both provide wider process windows with satisfactory etching characteristics, but we still believe that the process windows are too narrow and the etching characteristics can be further improved.
Hexafluoropropylene (C.sub.3 F.sub.6) has also been investigated by Wang et al. in the previously cited patents. It has the F/C ratio desired by Yanagida. However, the results show insufficient selectivity.
A theoretically promising etching gas is tetrafluoroethylene (C.sub.2 F.sub.4). However, it is considered dangerously explosive.
There are further considerations in selecting fluorocarbons for oxide etching. If a higher-order fluorocarbon is selected, a presently available commercial supply is greatly desired, even if a semiconductor grade needs to be developed. Furthermore, many of the higher-order fluorocarbons are liquids at near to room temperature. It is still possible to use liquid precursors by the use of bubblers to atomize the liquid in a carrier gas. However, bubblers present another expense, they need frequent maintenance, and the effective flow rate of the liquid precursor is difficult to tightly control. Gaseous precursors are much more preferred.
For these reasons, other fluorocarbon etching gases are desired.